1. Field of the Invention
The present invention relates to an optical disk device, and more particularly to optimization of power level for recording data.
2. Description of the Related Art
In an optical disk device such as a CD-R/RW drive and a DVD/R drive which allows recording of data, Optimum Power Control (OPC) and Running Optimum Power Control (ROPC) techniques are employed to optimize recording power level when recording data. OPC refers to a process in which test data are recorded (test-writing) on a predetermined area (PCA area) of an optical disk, and reproduced so as to determine a recording power level at which a value indicating reproduced signal quality, such as the β value, achieves a target value. ROPC refers to feedback control performed with respect to the recording power level such that the intensity of reflected light from the optical disk during recording of data is maintained at a constant level. This control is executed to overcome variations in recording sensitivity resulting from unevenness in application of the recording film on the surface of the optical disk, changes in laser beam shape due to a tilt of the optical disk, and offsets in tracking and focus.
FIG. 8 shows a change in the intensity of reflected light from the optical disk when a laser beam having a recording power is irradiated thereon. Immediately after the beginning of irradiation of the laser beam at the recording power level, the pit is not yet being formed, and the reflected light intensity is therefore high. However, after a while when the pit is formed, diffraction by the formed pit reduces the reflected light intensity to a constant level. This level is the reflected light intensity level (voltage level B) used in ROPC. In ROPC, adjustments are made such that the voltage level B of reflected light intensity, which is normalized with the recording power level, is maintained at a constant level. In actual processing, the following relationship is considered to hold true (refer to FIG. 9):Bo/Po=B/P=constant  (1)where Po denotes the optimized recording power level obtained through OPC, Bo denotes the level B value of reflected light intensity obtained when Po is used, P denotes a recording power level at a given position on the disk, and B denotes the level B value of reflected light intensity obtained when P is used. Based on the above relationship, the following equation is obtained:P=B/Bo·Po  (2)In accordance with this equation, the recording power P is adjusted by referring to the level B value of reflected light intensity.
Although the recording power P is thus conventionally adjusted under the assumption that the β value can be maintained at a constant value by employing the relationship B/P=constant, there frequently exist cases where such adjustment cannot actually maintain a constant β value over all regions of an optical disk, failing to achieve stable recording quality.
FIG. 10 shows an example of change in the β value generated when data is recorded while adjusting the recording power in accordance with the above equation (2). While the target β value βo can be attained at the inner portions of the disk proximate to the PCA area, the β value exceeds the target value βo at the disk outer peripheral portions, because of excessive adjustment (compensation) of the recording power. This indicates that the assumption B/P=constant does not always hold true, and that adjustment of recording power should be performed in accordance with some other appropriate relationship.